The invention relates to a method and a device for variably setting the braking force in a hydraulic brake system of a motor vehicle according to the preamble of claims 1 and 15, respectively.
DE 35 26 556 A1 discloses a brake system for motor vehicles which comprises a master cylinder which is designed as a tandem cylinder and is actuated by a brake pedal. Also provided is an auxiliary pressure supply system which comprises a hydraulic pump for producing the required brake pressure, and an electric motor actuating the pump, the hydraulic medium delivered by the pump being fed to the wheel braking device in order to produce the required braking force.
Printed publication DE 44 27 170 C1 discloses a method for ensuring the braking effect of brakes of vehicles in the event of moisture. For this purpose, a conclusion is drawn on the presence of moisture via a sensor device. If moisture is present, the time period is determined during which the brakes of the vehicle have not been actuated. If this time period overshoots a time threshold value, a braking operation is undertaken in which a vehicle retardation not perceptible to the driver is initiated. This braking operation vaporizes the water being precipitated on the brake and, the immediate and complete functionality of the brake is ensured. Moreover, DE 44 27 170 C1 discloses detecting the temperature of the brake parts and undertaking a braking operation for the case that the brake temperature undershoots a temperature limiting value. There is no disclosure of a determination of the friction factor of the brake disc/brake pad pairing nor of a compensation, based thereon, of a diminishing friction factor by means of increasing the brake pressure.
A further device for measuring and/or regulating the braking force in the brake system of a motor vehicle is disclosed in EP 0 189 082 A2. It is proposed in this printed publication to set the braking force of the wheel brakes so as not to overshoot a permissible maximum temperature value. This is intended to permit functioning continuous operation with a high, constant braking force. However, this device has the disadvantage that there is a need during the regulation to take account of relatively long time delays in the propagation of heat in the brake system, as a result of which the brake system to be regulated reacts relatively sluggishly, and maximum braking forces cannot be maintained continuously.
The publication PATENT ABSTRACTS OF JAPAN vol. 017, No. 430 (M-1460) discloses a device with the aid of which there is determined for the brake pressure a desired value which corresponds to the brake pressure which is to be set by the driver at the front wheels on the basis of the actuation of the brake pedal. The friction factor present for the brake disc/brake pad pairing is taken into account when calculating this desired value. This friction factor is determined as a function of the temperature and the rotational speed of the brake disc. No provision is made to increase the brake pressure in the case of an impermissible undershooting of the actual friction factor by comparison with the desired friction factor.
GB-A-2 297 134 A relates to a method and a device for controlling an ABS/PCS system. The aim in this case is to save entirely or partly on the vacuum brake busting, and to implement hydraulically the necessary brake busting by suitably driving a return pump and valves which are respectively included in the hydraulic unit. In addition to the implementation of pressure control in which the brake pressure in the wheel brake cylinders is set in accordance with the actuation of the brake pedal undertake by the driver, a function is also described with the aid of which a [inaudible] of the friction factor between the brake pads and brake discs, which is denoted as fading, is detected and appropriate counter measures are taken. Various evaluations are undertaken in order to be able to detect a diminution in the friction factor. Thus, a check is made as to whether the brake pressure prevailing in the master brake cylinder is higher than a threshold value; as to whether there is an actuation of the stop light switch, as to whether the ABS regulation is active; as to whether the speed of the vehicle is greater than zero or a threshold value; and as to whether the ABS system is operating properly. The pressure in the wheel brake cylinder is increased if a diminution in the friction factor is detected. The pressure is preferably raised in steps until the ABS regulations sets in at all wheels of the vehicle, or a maximum permissible prescribable pressure difference between the pressure of the wheel brake cylinder and the pressure in the master brake cylinder is reached. The determination of the actual friction factor of the brake disc/brake pad [inaudible], and an evaluation of the same with the aid of a desired friction factor are not disclosed. Also not disclosed is to increase the brake pressure to a value multiplied by a correction factor if the actual friction factor is within a defined friction factor stabilization range.
The following object emerges against this background: the aim is for a diminishing braking effect in hydraulic brake systems firstly to be detected simply and reliably, and secondly to be compensated in wide ranges without the brake system being overloaded in the process.
This object is achieved according to the invention with the aid of the features of claims 1 and 15, respectively.
The actual friction factor between the brake disc and brake pad of a wheel brake characterizes the conversion of the clamping force of the calliper into the retarding force on the brake disc. If the actual friction factor between the brake disc and brake pad impermissibly undershoots the desired friction factor, the brake pressure is stepped up, as a result of which a diminished brake power, which can arise as a consequence of overheating, wear or environmental influences such as moisture, pollution or the formation of ice, can be at least partially compensated such that no worsening of the brake power occurs, either subjectively or objectively. In order to prevent over-stressing of the brake system and brake fading resulting therefrom, stressing of the brake system and brake fading resulting therefrom, or to prevent a possible brake failure as a consequence of the increase in brake pressure carried out automatically, as an additional condition before the brake pressure increase, a check is made as to whether the actual friction factor is within a defined stabilization range which comprises a band of friction factors below the desired friction factor marking a maximum. The increase in brake pressure is carried out if the actual friction factor lies within this stabilization range. If the actual friction factor lies below the stabilization range, for reasons of overload protection it is not expedient to carry out an increase in brake pressure; in this case the brake pressure increase does not take place, and fault or alarm signal is displayed which informs the driver of the poor brake condition.
The stabilization range can be permanently prescribed, or it can be determined in the course of operation with the aid of changing state variables or performance quantities such as, for example, maximum achievable retardation, temperature of the brake system, wetting of the brake parts with moisture, etc., a high measure of flexibility being achieved thereby.
In an advantageous development, the stabilization range is subdivided into a constant range, in which a constant, maximum braking retardation can be maintained over deteriorating friction factors, and into a gradient range in which although the brake pressure is piled the maximum braking retardation is not achieved. The constant range in this case comprises higher friction factors than the gradient range and the two ranges together expediently fill up the stabilization range completely. The boundary between the constant range and gradient rangexe2x80x94the lower friction factor of the constant rangexe2x80x94is advantageously variably set. With increasing temperature of the wheel-braking device, it is particularly expedient to displace the lower friction factor of the constant range in the direction of the lower limit of the gradient range, as a result of which the constant range is expanded at the expense of the gradient range. The result of this displacement of the lower friction factor of the constant range is to achieve a constant retardation over an increasing range of small friction factors without displacing the lower limit, serving the functional reliability, of the stabilization range further in the direction of smaller friction factors.
In order to compensate diminishing braking forces, the brake pressure is operated on by a correction factor which is greater than one and which is expediently calculated as a function of the measured or computed actual friction value, the desired friction value and the lower limiting value of the current rangexe2x80x94the gradient range or the constant range. The correction factor advantageously rises linearly in this case in the constant range, starting from the value 1, with decreasing friction factors, and subsequently drops in the gradient range with decreasing friction factors in a continuous and linear fashion until it reaches the value 1 for the lower friction factor of the gradient range.
The correction factor becomes ever larger in the constant range, the smaller the lower friction factor of the constant range becomes, that is to say the more the lower friction factor of the constant range is displaced in the direction of the lower limit of the gradient range. It is thereby possible to take account of the fact that an increasing brake pressure is required with increasing temperature in order to compensate the temperature-induced brake fading.